In recent years, in order to improve usability for operators and to improve accuracy in diagnostics, there have been proposed a variety of optical systems to be applied to medical endoscopes which performs focusing to provide observation in a wide range from high-magnification observation at close range to far distance observation by.
As examples of such endoscope optical systems, PTL 1 to PTL 4 disclose optical systems which are all composed of three groups of positive, negative and positive groups and in which focusing is performed by the second group moving along an optical axis.